The present invention relates to ophthalmic lenses, and more particularly to ophthalmic lenses which are photochromic, that is, those in which the colouration, or more precisely the absorption spectrum, varies in a reversible manner under the action of radiation, more particularly ultraviolet radiation, which they receive.
As is known, ophthalmic lenses are currently produced either from mineral glass or from organic material.
The organic material which is most commonly used in the latter case is the monomer known under the tradename "CR 39"; this monomer is ethylene glycol diallyl dicarbonate and this is polymerised with the aid of a catalyst, for example isopropyl percarbonate.
Compared with ophthalmic lenses made of mineral glass, opthalmic lenses made from this type of organic material have the advantage of being lighter, other conditions being equal; they have the additional advantage of being unbreakable.
In order to render ophthalmic lenses made of organic material photochromic, two methods are currently envisaged.
The first method, analogous to that which is already proving satisfactory in the case of ophthalmic lenses made of mineral glass, consists in incorporating photochromic pigments into the lenses in question.
However, this solution has not yet proved commercially satisfactory for lenses made of organic material; in fact, after polymerisation of the organic material into which they are incorporated, the photochromic pigments tested hitherto do not retain their photochromic property, in particular because the catalyst required for this polymerisation causes inhibition of the pigments.
Thus, for chemical reasons, this first solution has not in practice led to stable photochromic ophthalmic lenses made of an organic material.
The second method envisaged for rendering an ophthalmic lens, made of an organic material, photochromic consists in embedding a layer of photochromic mineral glass in the bulk of the latter.
The experiments carried out in this direction hitherto have concerned the use of a layer of photochromic mineral glass in the form of a film, that is to say a layer having a massive structure with a well-defined coefficient of expansion which depends on the nature of the mineral glass used.
Again, these experiments have not led to commercially satisfactory results because of possible splitting between the layer of photochromic mineral glass and the mass of organic material in which it is embedded, in particular during the stress release which inevitably occurs when the ophthalmic lenses made in this way are routed, this routing being necessary in order to match the lenses to the spectacle frames in which they are to be mounted. This possible splitting is generally due to the contraction of the organic material in question during its polymerisation, and to the differences in coefficients of expansion between the organic material and the photochromic mineral glass used.
Thus, in this case, it is for mechanical reasons that, hitherto, this second solution has not in practice led to stable photochromic ophthalmic lenses made of organic material.
Therefore, at the present time, commercial photochromic ophthalmic lenses are usually made of mineral glass.
It is an object of the present invention to make it possible to produce photochromic ophthalmic lenses from organic material.